Brillouin scattering studies of the ferroelectric phase transition in L-lysine doped TGS single crystals

The Brillouin spectroscopy study of triglycine sulphate crystals doped with L-lysine (TGSL10) has been performed. We observed longitudinal or quasi-longitudinal acoustic phonons propagating parallel to b, c and c p (c p denotes direction perpendicular to the ab plane) crystal’s axes. The temperature dependence of both frequency shift and the spectral line width have been investigated in temperature range 25–75 8C. Our results are similar to those obtained for pure TGS. The spectra of the acoustic modes propagating in c and c p directions show step-like anomaly in frequency shift and an anomalous broadening of the line just below the transition temperature. This behaviour is in contrast to ‘the temperature independent’ quasi-longitudinal phonon propagating parallel to spontaneous polarization (b-axis). The obtained results can be explained by (characteristic in the ferroelectric phase transition) influence of depolarization field effect of the long range dipole–dipole interaction. The relaxation time of polarization fluctuations in the vicinity of the phase transition has been estimated. The increase of the speed of relaxation process caused by lysine doping has been revealed. q 2004 Elsevier B.V. All rights reserved.